In the optical communication field, there is used an arrayed waveguide grating as illustrated in a diagram of FIG. 11, for example. The arrayed waveguide grating has one or more optical input waveguides 2 arranged side-by-side (plural optical input waveguides are shown in the figure), a first slab waveguide 3 connected to output sides of the optical input waveguides 2, an arrayed waveguide 4 connected to the output side of the first slab waveguide 3, a second slab waveguide 5 connected to the output side of the arrayed waveguide 4 and one or more optical output waveguides 6 arranged side-by-side (plural optical output waveguides are shown in the figure).
The arrayed waveguide 4 is provided for propagating light output from the first slab waveguide 3 and has a plurality of waveguides (channel waveguides) 40 arranged side-by-side. Adjacent channel waveguides 40 are different in length by a predetermined length (ΔL) and the arrayed waveguide 4 gives each signal a phase difference in the arrayed waveguide grating 11.
Typically, the arrayed waveguide 4 includes a large number of channel waveguides 40 or, for example, 100 waveguides, however in the figure, a small number of waveguides 40 are only shown for easy illustration.
In the arrayed waveguide grating 11, for example, when a wavelength division multiplexed optical signal comprising signals having wavelengths λ1, λ2, λ3, . . . , λn enters one optical input waveguide 2, this signal passes through the optical input waveguide 2 into the first slab waveguide 3. Then, the signal is diffracted and spread by the first slab waveguide 3 and is transmitted to the arrayed waveguide 4 to propagate therethrough.
After passing through the arrayed waveguide 4, the signals enter the second slab waveguide 5, converge on and then are output from optical output waveguides 6. As the channel waveguides 40 of the arrayed waveguide 4 are all different in length, a phase difference appears in each of the signals that have passed through the arrayed waveguide 4. Due to this phase difference, the wave fronts of the signals tilt and this tilt angle determines focal points of the signals.
For this reason, the focal points of the signals having different wavelengths differ from each other and accordingly the optical output waveguides 6 are formed at the respective focal points. With this configuration, the signals of different wavelengths are extracted by the optical output waveguides 6, respectively, thereby completing the function as a wavelength-division demultiplexer of the arrayed waveguide grating.
Moreover, as the arrayed waveguide grating takes advantage of the principle of reversibility of the optical circuit, the arrayed waveguide grating also handles the function as a wavelength-division multiplexer as well as a wavelength-division demultiplexer. That is, reversing the above-described procedure, when signals having differing wavelengths λ1, λ2, λ3, . . . , λn enter respective optical output waveguides 6, the signals passes through the above-mentioned propagation path in reverse, the signals are multiplexed by the second slab waveguide 5, the arrayed waveguide 4 and the first slab waveguide 3 and output from one optical input waveguide 2.
Generally, as the arrayed waveguide grating is made of silica-based glass, there occurs temperature fluctuation due to the temperature-dependent refractive index of the silica-based material. Specifically, as ambient temperature changes where the arrayed waveguide grating is placed, the light transmission center wavelength (center wavelength) of the arrayed waveguide grating changes dependently on the temperature, causing a shift of the center wavelength of about 0.8 nm over the general operating temperature range (−5° C. through 70° C.). In view of this, there was the need to control the temperature of the arrayed waveguide grating chip as a whole with use of Peltier elements or heaters.
Thus, there was a great demand for temperature independence (insensitivity) of the arrayed waveguide grating, and recently, the technique of compensating the temperature dependence of the center wavelength has been developed (development of an athermal arrayed waveguide grating). This technique has realized temperature-control-free arrayed waveguide gratings and no electric power supply (see non-patent documents 1 and 2).
FIG. 12 illustratively shows an example of a temperature-independent arrayed waveguide grating that was proposed conventionally. In this arrayed waveguide grating, a first slab waveguide 3 is cut at division surfaces 8 in a plane crossing the path of light passing through the slab waveguide 3 to constitute divided slab waveguides 3a and 3b. Moreover, the arrayed waveguide grating is also divided at surfaces 18 crossing the division surfaces 8 into a chip 1a and a chip 1b. 
Further, there is provided a sliding member 7 for temperature-dependently moving the divided slab waveguide 3a along the division surface 8 in such a direction as to reduce the temperature dependence of the light transmission center wavelength of the arrayed waveguide grating (direction of the arrow A or B). Movement of this divided slab waveguide 3a by the sliding member 7 is utilized for realization of temperature independence of the light transmission center wavelength of the arrayed waveguide grating.
Non-patent document 1: Y. Inoue, A. Kaneko, F. Hanawa, H. Takahashi, K. Hattori, S. Sumida, “Athermal silica-based arrayed-waveguide grating multiplexer,” Electron Lett. ,vol. 33, pp. 1945-1947, 1997
Non-patent document 2: T. Saito, K. Nara, Y. Nekado, J. Hasegawa, K. Kashihara, “100 GHz-32 ch athermal AWG with extremely low temperature dependency of center wavelength,” OFC'03 MF47, pp. 57-59, 2003.